Air filters are used for many different purposes in many different fields. Two different air filter categories are high efficiency particulate air (HEPA) filters which are used in clean room technology, and the other category, gas cleaning filters are used in emissions control and also in vacuum cleaners. HEPA filters usually are able to contain 99.97% of all particles over 0.1 µm in diameter. Gas filters accuracy varies between 0-100% efficiency with particles sizes between 0.3 µm and 100 µm. The fractional penetration measurement and the measurement of the pressure drop is key for describing the characterization and quality control for both filters. To test this a procedure was developed and two test rigs were built. By simultaneously measuring the fractional penetration and the pressure drop, loading studies can be done in a few mlnutes. Face velocity, upstream concentration, test dust material, filter material and electrostatic charge of the particles and fitter fibres can show influence under varying conditions. The study goes on to discuss the time and size dependent penetration and pressure drop of electret fibre, glass fibre, and paper media.